Telephone carriers are rapidly deploying digital Internet Protocol Television (IPTV) service over Fiber to the Node (FTTN) technology. These carriers are utilizing Digital Subscriber Line (DSL) technology such as, for example, ADSL2+ or VDSL2 to modulate and carry the digital IPTV over existing copper pairs. FIG. 1 illustrates portions of a typical telecommunications system used to deploy IPTV service using VDSL technology. The telecommunications system includes feeder cables 21, often called F1 cables, which are in communication with distribution cables 22, often called F2 cables, through a cable cross-connect box 30. Although a typical telecommunications system includes several feeder cables 21 and several distribution cables 22, only one feeder cable 21 and one distribution cable 22 is illustrated. The VDSL system includes a fiber fed node 26, fed by a fiber cable 20, a DSLAM 28 and a cross-connect box 30. A ground connection 29 is provided at the DSLAM 28 and a ground connection 31 is provided at the cross-connect box 30.
The IPTV carriers position the fiber fed node 26 next to a cross-connect box 30. The fiber cable 20 carries signals to and from the fiber fed node 26. Copper jumper cables 32 are used to transmit the signals between the fiber node 26 and the cross-connect box 30. The distribution cables 22 extend from the cross-connect box 30 and transmit signals between the cross-connect box 30 and the network interface device 34 at the subscriber's premise 36. Although a plurality of subscriber premises 36 are typically provided along the distribution cable 22, only one is illustrated in FIG. 1.
As shown in FIG. 2 the distribution cable 22 includes a number of twisted copper pairs 40 (one of which is shown), a cable shield 42 around the pairs 40, and an insulative sheath 44 around the shield 42 Each pair 40 includes a first conductor or tip 46 and a second conductor or ring 48. Referring back to FIG. 1, two pairs of conductors 40 within the distribution cable 22 are shown. A pair 40a of conductors is terminated at the subscriber's premise 36 and other pairs, such as pair 40b, extend beyond the subscriber's premise and are terminated beyond the subscriber's premise.
As illustrated in FIG. 1, the DSLAM 28 includes a DSL modem 50 which communicates with a remote DSL modem 52 located in the network interface device 34 (NID). Although the NID 34 is shown outside of the subscriber's premise 36, alternatively, the NID 34 and remote DSL 52 modem can be located inside the subscriber's premise 36. The interconnection cables 32 carry signals between the DSLAM 26 and the cross-connect-box 30. Digital signals from the fiber feeder cables 20 are routed through the modem 50 of the DSLAM 28. The modem 50 converts the digital IPTV signals to analog DSL signals and passes the analog signals to the twisted pairs 40 of the distribution cables 22. The distribution cables 22 carry the DSL signals the remaining distance to the subscriber premise 36, often termed the “last mile” from the DSLAM 28 to the NID 34. At the subscriber's premise 36 the remote DSL modem 52 converts the analog DSL signals back to digital signals.
A number of access points/access locations 38 are also provided along the distribution cable 22, and can be located in the pedestals as illustrated. Other types of access points, such as for example sidewalk boxes or hand hole accesses in buried cables, or aerial terminals in aerial cable can also be provided. Although not illustrated in order to provide clarity to FIG. 1, each of the twisted pairs in distribution cable 22 extends through the access points 38 spaced along the cable 22 as indicated by the row of pedestals 38 and power grounding illustrated above cable 22. Extension of the cable 22 through the access points 38 is illustrated in FIGS. 3 and 4. A section 58a of the distribution cable 22 extends from a first access point 38a to a second access point 38b. A first end of cable section 58a is provided in an access point 38a and a second end of cable section 58a is provided in the another access point 38b. Sections 58b and 58c of the cable 22 are adjacent section 58a. An end of cable section 58b extends within the first access point 38a and an end of cable section 58c extends within the second access point 38b. 
As shown in FIGS. 1 and 3, the distribution cables 22 of the telecommunications system 10 are often positioned proximate power lines 54. The cable sections located proximate the power lines are often described as “exposed” to the power lines 54. Magnetic fields (represented by the circles 60 encompassing the power lines 54 and the distribution cable 22) from these nearby power lines 54 cut through the distribution cable 22, as do fields from many other sources including radio transmitters. As illustrated in FIG. 3, these magnetic fields 60 induce longitudinal AC voltages, i.e. noise, into the cable section 58a of the distribution cable 22 and onto the twisted pairs 40. The induced longitudinal voltages cause one end 62 of a pair 40 (for example, the end proximate the first pedestal 38a) to have voltage with respect to an opposite end 64 of the pair 40 (for example, the end proximate the second pedestal 38b of the pair). If a ground connection 66 is provided from the pair 40 within the first access point 38a to a nearby power neutral 56 in order to ground the pair 40, the opposite end 64 of the pair 40 of the cable section 58 will show AC voltage with respect to ground. A test instrument 68, such as a voltmeter, can be utilized to measure the AC voltage on the pair 40 with respect to ground. A first lead 69 of the test instrument 68 is connected to the pair 40 at the second end of the cable section 58a and the second lead 71 of the test instrument 68 is connected to ground 56 to measure the AC voltage on the pair 40 with respect to ground. The longitudinally induced voltages also appear on the cable shield 42 when the shield is open, as illustrated in FIG. 3 by an open bond between a first location 73a and a second location 73b. The voltage on the open shield section can also be measured with voltmeter 68 by connecting the first lead 69 of the voltmeter to the open bond at the first location 73a and by connecting the second lead 71 to the ground 56.
To minimize longitudinal voltage on the pairs 40, the shield voltage must be shorted out. In order to short out the shield voltage, the opposite ends of the shield 42 of the section 58a of the distribution cable 22 must be connected together, external to the distribution cable 22, through a low resistance path so as to have very low voltage between the ends of the shield 42 of the cable section 58a in the presence of shield current flow. Although shield ground can be used to short out the shield voltage, as noted above, the shield ground must have a low resistance to be effective and typically shield grounds such as ground rods used at the shield are normally too high in resistance to be effective. The neutral 56 of the parallel power lines 54, however, have a much lower resistance and can provide an effective shield ground. The power neutral 56 consists of multiple grounds located throughout the system connected together by the neutral conductor 55. This power neutral system is commonly called the Multi Ground Neutral, MGN.
The use of shield grounding to cancel the induced voltage in the distribution cable 22 is also illustrated in. FIG. 4. As illustrated in FIG. 4, a shield bond 73 is provided between first location 73a and second location 73b within the access point 38b. A power bond 74 in connection with the shield bond 73 provides an effective low resistance shield ground. In order to utilize the power bond 74 as an effective low resistance shield ground, it is important that the power bond 74 extending from telephone cable shield 42 to power neutral 56 and to other low impedance grounds be maintained in addition to maintaining good shield continuity. As shown, section 58a of the distribution cable 22 extends from a first access point 38a to a second access point 38b and has a first end 70 proximate the first access point 38a and a second end 72 proximate the second access point 38b. The shield 42 of the section 58a includes a first end 42a and a second end 42b. The first end 42a of the shield is bonded through a ground bond 74 to the power neutral 56. The second end 42b of the shield of section 58a is bonded through a ground bond 74 to a second power neutral 56. These ground bonds 74 provide an external low resistance connection between the opposite ends 42a, 42b of the shield section. When the first and second ends 42a, 42b of the shield 42 of the cable section 58a of the distribution cable 22 are grounded as described above, a current will flow through the cable shield 42 of the section 58a which circulates hack through the power neutral within that section 58a. This induced shield current produces its own opposing magnetic field, illustrated by the circles 80. The induced current on a properly bonded and grounded shield 42 of the distribution cable section 58a tends to cancel the induced longitudinal noise voltage on the cable pairs 40 in that section 58a of cable 22. The induced shield current on cable section 58a in FIG. 4 can be measured with instrument 77. In contrast, no shield current is flowing in cable section 58a shown in FIG. 3 due to the open in the shield bond between locations 73a and 73b. In addition, due to cancellation of induced noise voltage on the pair 40 in FIG. 4, a low voltage will be measured on the pair 40 by the voltmeter 68 compared to the higher voltage measured on the pair in FIG. 3.
DSL signals are provided at a much high frequency than POTS signals, allowing the DSL signals to be superimposed over a twisted pair 40 carrying analog POTS service without interference between the POTS service and the DSL service. DSL circuits are susceptible to high frequency interference, and field experience has shown that the IPTV circuits may not be reliable unless the cable shields 42 are bonded and grounded all the way from the fiber node DSLAM 28 to the subscriber's premise 36. Cancellation of the longitudinally induced noise voltage in the manner described is more effective at higher frequencies, e.g. frequencies relating to the xDSL bands, than at lower frequencies, e.g. 60 Hz and frequencies relating to the POTS band. Thus, in xDSL circuits, good shield continuity and good shield grounding are critical for xDSL circuits.
Referring to FIG. 1 (with the understanding that although not shown, the cable 22 extends within the access points 38), multiple shield bond points 80 are used to create many sections of the distribution cable 22, allowing cancellation of voltages induced on the pairs 40 within each section. Each section may have a different exposure to the power lines 54. If the shield current on one section having a certain exposure to the power lines 54 is allowed to enter another section having a different power exposure, the shield current may increase the induced longitudinal voltages on the pair 40 rather than cancel the longitudinal voltage on the pairs 40. Therefore, a bond 74 to power neutral is provided at each instance where the power exposure changes.
In some instances, a bad bond 74 or an open shield 42 keeps the induced shield current from flowing to an adjacent section(s) of the distribution cable. FIG. 5 illustrates an example of this situation. Multiple access points 38 (identified as 1-8) are provided along the distribution cable 22, providing sections of the distribution cable 22 identified as sections 58a-58g. At each access point, a power bond 74 is provided to the cable shield 42 to cancel induced voltages. A break/open 82, however, is present in the shield 42 of the distribution cable 22 within the section 58d. As a result of the break/open 82, shield current is not provided on the shield 42 of section 58d. As discussed above, the power lines 54 induce longitudinal voltages onto the pairs 40 of the distribution cable 22. Although these longitudinal voltages will be cancelled in sections 58a, 58b, 58c, 58e, 58f, and 58g due to the shield current in those sections, the longitudinal voltages occurring in section 58d will, however, remain un-canceled due to the shield open 82. The un-canceled longitudinal voltages drive cable distributed capacitance-to-ground to cause longitudinal currents in the pairs 40. If the resistances of the tip and ring of the pair 40 are not precisely equal, i.e. balanced, these longitudinal currents induce metallic voltages into the pair 40. Or, if tip and ring capacitances-to-ground are not precisely equal, the longitudinal voltage is capacitively coupled metallic into the pair. These longitudinal voltages and longitudinal currents cause pixelization and frame freeze of the IPTV service at the subscriber's premise 36.
To avoid interruption or pixelization, which can occur for example, when the local ham radio operator decides to key his transmitter or when the power system load changes, cable manufacturers spend considerable effort to keep tip and ring resistances and capacitances equal (balanced) in order to maximize the amount of longitudinal current/voltage cancelled. Pair balance cannot however, be depended upon to reduce the metallic voltage on a pair by more than 60 dB below longitudinal voltage in the POTS, ADSL or VDSL bands. Therefore, in addition to achieving good balance, the longitudinal voltages coupling onto the pairs must be minimized by providing good shield bonding. By providing good shield bonding for cables carrying IPTV service, longitudinal voltages, which are measured as noise-to-ground, will be minimized thereby reducing the occurrence of pixelization and frame freeze, for example. In order to deploy IPTV over existing distribution cables, therefore, carriers must clean up all the shield bonding from the cross-connect box 30 to the subscriber's premise 36. To deliver IPTV to the premise 36 the process must focus on the noise performance of distribution cables 22 between the cross-connect box 30 and the furthest IPTV subscriber's premise 36 along the distribution cable 22 in question. In some instances, attention must also be given to bad bonding in the distribution cable 22 beyond the furthest IPTV subscriber's premise 36, i.e. a portion of the distribution cable beyond the last IPTV subscriber's premise 36 which carries only POTS service, as high noise-to-ground can back-feed longitudinal voltage on other pairs, and cause trouble in the IPTV sections. In addition, noise is coupled from the F1 feeder cables 20, to the F2 distribution cables 22 by jumpers 32 in the cross-connect box 30. In locations with excessive power influence (PI) or noise-to-ground (Ng) in the feeder cables 20, the feeder cable shield bonding will also need to be repaired before IPTV can be provided on the cable. Thus, there is a need for a simplified method of locating open cable shields that can be easily understood by a broadband technician.
As mentioned above, ADSL and VDSL technology is used to modulate and carry the digital IPTV signals. FIG. 1 illustrates the connections utilized for VDSL and ADSL technology. With VDSL technology, pairs with POTS signal come from the central office 23 through feed cable 20 to the cross-connect box 30, where a first jumper connects through the interconnect 32 to the DSLAM 28. There the pair is looped through modem 50, and is returned by a second jumper through the interconnect cable 32 back to the cross-connect 30, then into the distribution cable 22 where it travels to the subscriber 36.
It is to be understood that the same concepts apply with respect to ADSL technology. In particular it is noted in FIG. 1 that with ADSL technology, the ADSL DSLAM 25 is provided at the central office 23 and the DLSAM in the fiber node 26 is not utilized. Pairs with POTS signals and ADSL signals superimposed come from the central office 23 through feeder cable 21 to the cross-connect box 30, where a single jumper connects each pair directly to the distribution cable 22 where it travels to the subscriber 36.
A method currently used to locate open shield sections utilizes modem sync rate readings. This method is not, however, accurate. The broad band technician cannot accurately locate the open shield section by taking xDSL modem sync rate readings at access points along the cable because as the technician approaches the DRAM, the pair loss decreases and the signal is stronger. An acceptable sync rate at an access point, therefore, does not mean the bad bonding is behind you, it just means the pair has an acceptable signal-to-noise ratio at that location. The source of the noise from the bad shielding can be either before or after this location. Because, this modem sync rate method can not be used to accurately predict the location of the open shield, there is a need for a method for isolating an open shield section that is more accurate.
Another disadvantage of the modem sync rate method is that the technician must open the pair to take a sync rate reading at an access point along the cable. When the pair is opened, the technician disrupts the induced longitudinal current flow on the pair under test, changing the noise level metallic on the pair and the sync rate. Thus there is a need for a shield bonding isolation method that does not require the pair to be opened at each access point.
Measuring noise metallic (Nm, i.e., the noise between the two conductors, of the pair) can also provide an inaccurate indication of noise-to-ground (Ng, i.e. the noise from conductors to ground) which can be misleading. For example, if the technician happens to pick a pair with above average balance, the test may indicate an acceptable noise metallic (Nm) level when in fact, the noise-to-ground (Ng) level is unacceptable. Therefore, other pairs in the cable with an average balance and which provide excessive noise-to-ground levels will go undetected unless that particular pair is measured. Thus, there is a need for a shield bonding isolation method that does not depend on pair balance.
Often the cable is spliced at points along the run. Many of these splices are direct buried, and it is very costly to open them unless they are truly in trouble. The broadband technician will need a way to isolate the general areas of noise intrusion due to poorly bonded shield sections to focus shield testing and repair efforts to a limited number of splices or terminals in these areas.
The present invention provides an apparatus and method for testing the integrity of the cable shield and if necessary isolating the shield trouble. The apparatus and methods of the present invention overcomes the problems presented in the prior art and provide additional advantages over the prior art, such advantages will become clear upon a reading of the attached specification in combination with a study of the drawings.